All cancer therapies must be based on differences between tumor cells and the normal tissue from which they arise. One obvious, but largely unexploited, difference between tumor cells and normal cells is aneuploidy. Aneuploidy (abnormal chromosome number), including near polyploidy (increased genome sets), is a prominent feature of most cancers yet the consequences of this alteration of the genome is poorly understood. Aneuploidy can either be due to abnormal numbers of whole chromosomes, which originate from mitotic chromosome segregation errors, or to structural rearrangements of chromosomes, which originate from DNA breaks and recombination. This laboratory has focused on whole chromosome aneuploidy because it is one of the most mysterious aspects of tumor biology; there has been a one hundred year debate over whole chromosome aneuploidy: Is it beneficial, detrimental, or a completely neutral passenger phenomenon during tumor development? The uncertainty about the contribution of aneuploidy to tumorigenesis is largely fueled by the paucity of mechanisms explaining how aneuploidy impacts tumorigenesis or the properties of mature cancers. This grant will address two key questions about aneuploidy and cancer. In the first two aims, I will characterize potential mechanisms by which whole chromosome aneuploidy could initiate/promote cancer. The first aim will address whether chromosome segregation errors can produce DNA damage. A series of imaging and biochemical experiments will test the hypothesis that micronuclei, generated by whole chromosome mis-segregation, have abnormal chromatin and/or nuclear architecture that leads to defective DNA replication and chromosome breaks. The second aim will define the role of a specific recurrent aneuploidy - extra copies of chromosome 8 - in tumor development. Although polysomy occurs in many tumor types, the experiments will focus on the development of acute myelogenous leukemia because of methods to generate myeloid progenitor cells that do or do not contain polysomy 8. In the third Aim, we will determine if polysomy 8 causes vulnerabilities that can be exploited to discover new cancer drug targets. This third aim follows directly from previous work under this grant that defined 'ploidy-specific lethality', where genes that are not essential in normal diploid cells become essential in polyploid cells or extra centrosome-containing cells that have a chromosomal instability phenotype.